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The telescope requirements include slewing and offsetting time, trajectory rate limits, tracking accuracy, wind disturbances rejection and the avoidance of resonances excitation.
To evaluate the Control System behavior, a State Space Matrix has been estimated from the modal analysis of a finite element model of the complete telescope, including also the pier, in the “free rotor” condition.
The plant transfer functions of the Azimuth and Altitude axes have been analyzed to evaluate the more critical resonances that can affect the control loops bandwidth. The velocity and the position loops architectures have been designed and tuned to evaluate how the control bandwidth influences the structural resonances.
Different loops architectures have been implemented to compare the results, also including feedforward control to enhance the tracking performance and low pass filters to minimize the structural modes excitation. The control design results are presented.
A telescope model, including Azimuth and Altitude axes, frictions and motor torque disturbances, encoders quantization, loops sampling and latencies, has been created. The wind disturbance has been implemented as a time-varying force acting directly on the telescope structure, generated using a velocity time history with the requested PSD.
Several simulations, here presented, with and without the wind disturbance, have been done to analyze the performances respect to all the requirements and to assess the structural behavior. The simulations consider the axes moving at the same time to evaluate the cross coupling effect following all the foreseen trajectories.
The dome and telescope (DMS) are designed to ensure high performance in one of the most seismic active areas in the world, Cerro Armazones in Chile.
The Dome diameter is 86 m and sits on the top of a stiff concrete pier, which has been designed with horizontal seismic devices to reduce the seismic accelerations on the structures. The isolation system consists of a combination of High Damping Rubber Bearings (HDRBs) and lubricated spherical bearings.
The Telescope structure has been designed to be adaptive, during operation it is extremely stiff with low damping to guarantee the pointing and tracking (fixed condition) and when subjected to strong earthquakes it is flexible with high damping (isolated condition) to reduce the accelerations on the mirrors and instruments.
To fulfill these requirements, a 3D adaptive seismic isolation system has been designed with unique features. The telescope natural frequencies and damping change suddenly when the telescope is subjected to a 1-year return period seismic event, which is the maximum threshold acceleration acceptable without isolation. In the isolated configuration, the telescope frequencies range between 0.3 Hz (isolation frequency) and 30 Hz (highest frequency of interest), while in the fixed configurations the frequencies range between 2.6 Hz and 30Hz.
The vertical and horizontal acceleration reduction is obtained with special devices designed for this type of applications.
This paper presents the design and shows the results of the sophisticated nonlinear time history analyses performed on the DMS. The large finite element models consist of about 75000 nodes and over 110000 elements and include nonlinear spring damper elements calibrated experimentally to model the vertical and horizontal behavior of the seismic devices.
The performance check of the preliminary design of the ELT has been completed through a variety of analyses: structural static analysis (FEM), modal and harmonic analysis (FEM), seismic spectrum and transient analysis (FEM), wind analysis (CFD and Wind Tunnel Test), thermal analysis (FEM), vibration analysis (FEM + State Space model), Servo analysis for pointing and tracking (State Space model).
A FEM model of the telescope has been created to analyze the telescope behavior against all the significant actions: gravity, wind, seism, thermal, manufacturing and alignment errors. The model includes the telescope pier and the pier foundations.
A Wind Tunnel Test campaign has been carried out on a scaled model of the Dome and Telescope to assess the wind action on the structures. The campaign has been supported by a detailed CFD analysis with several cases of Dome orientation, Dome configuration, wind velocity and turbulence intensity.
A State Space model of the telescope has been set up to perform the Servo analysis of the azimuth and altitude control system. A comprehensive State Space model of the Dome, the Ground, and the Main Structure has been set up to perform the vibration analysis of the whole observatory (including the machinery in the auxiliary building and the erratic vibrations from the ground).
The present paper provides a synthetic description of the generated models and the most significant results.
The bearing structure of the Dome is a truss structure made of steel, having a base ring and a series of arch girders as its main elements.
The Dome Rotation is performed by 36 trolleys, which are fixed to the top of the reinforced concrete Dome Base. Safety against seismic events is guaranteed by a dedicated Isolation and Damping System at the Dome Pier.
The Dome is covered by a custom Cladding System, that has been tailored in order to provide the required thermal insulation and withstand the harsh Environmental Conditions of the ELT Site.
With the aim of controlling the airflow around the Telescope, the ELT Dome is provided with a series of 89 Louvers, which are distributed among the rotating and the fixed structures. Besides, a Windscreen made of four permeable aluminum panels protects the Telescope; each panel spans over the 42m slit and is 10m high. The Windscreen is able to track with the Telescope on a 20 to 70deg range of the altitude angle.
The Auxiliary Building is a ring surrounding the Dome Pier and houses the Dome Technical Rooms, thus guaranteeing a radial distribution of all the Services. Among the Dome Supplies, a custom HVAC System is able to control the Telescope Chamber temperature with a ±2°C precision. The ELT Dome is provided with specific Plants so to supply Power to all the relevant loads and to the Electrical Equipment, as well as with a custom Global Control System and a series of Safety Systems.
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